Carbon dioxide recovery plants are widely used to clean and/or recover carbon dioxide released e.g. from combustion of hydrocarbons, fermentation and gas processing. The recovered carbon dioxide can optionally be liquefied and sold as an end-product or utilized in the production of a given plant.
In a typical absorber-regenerator system, recovery/removal of carbon dioxide is performed by introduction of the feed gas in an absorber, where the gas contacts a lean solvent flowing down the absorber. The carbon dioxide is at least partially absorbed by the lean solvent and the carbon dioxide depleted gas leaves the absorber for further processing or discharge.
The solvent containing the carbon dioxide is then treated to release the carbon dioxide from the absorbent most often by stripping, and the carbon dioxide may be recovered or further purified and the absorbent is reused to remove further carbon dioxide from an incoming gas.
Conventional technologies available for recovering the absorbent and carbon dioxide, respectively, include stripping. The absorber-regeneration system typically allows continuous operation for recovery of carbon dioxide.
In designing processes and determining parameters resulting in the required high purity carbon dioxide and at the same time at the highest rate of product yield, the further downstream purification steps often comprise open and closed loop systems, wherein the lean streams of absorbent, which may still contain remnants of carbon dioxide, are further processed and recycled to extract even more carbon dioxide from the absorbent. Implementing such loop systems thereby facilitates the recovery and reuse of streams, such as absorbent and/or water, in order to reduce costs and waste.
However, the further processing for regeneration of absorbent or extracting more carbon dioxide from the absorbent also requires additional energy, such as for cooling, heating and pressurising. In general, the energy consumption required per unit yield increases with the purity of the absorbent, i.e. the energy consumption required for recovering the last remnants of carbon dioxide from a lean stream of absorbent is higher per unit yield as compared to the recovery of the first units of e.g. a carbon dioxide-rich absorbent stream.
Several plants for the recovery of carbon dioxide improving on the overall energy efficiency have been described. US 2013/0055756 relates to such a recovery plant, wherein lean absorbent from the regenerator tower is recirculated to the top of the regenerator tower using conventional reboiling, and the mixed stripper off gas is compressed and condensed to recirculate solvent to the regenerator tower for further recovery. An intermediate condensing heat exchanging step is included where the lean absorbent and the mixed gas are heat exchanged. The energy efficiency however is in the heat pump system, 6, located in and in between the absorption tower and the regeneration tower.
WO 2004/005818 describes a method for regeneration of absorbent wherein a lean and semi-lean absorbents are generated by passing the loaded absorbent through a first regenerator providing a lean absorbent and a second regenerator providing a semi lean absorbent where low pressure steam to at least the first regenerator is provided by steam generated in the same or another process.
WO 2008/063082 also describes a method for regeneration of absorbed carbon dioxide, wherein heat energy is recovered from the carbon dioxide gas. An absorbed carbon dioxide stream is subjected to a stripping procedure thereby creating a heated gaseous carbon dioxide-rich stream and a liquid carbon dioxide-lean absorbent stream. The heated gaseous carbon dioxide-rich stream is subjected to a number of compression steps with intermittent addition of cooling water, and heat is recovered from the compressed gas using a heat exchanger. The recovered energy may be used for heating the liquid carbon dioxide-lean absorbent stream in a regeneration reboiler.
Thus, energy is recovered from the compressed heated carbon dioxide and this heat may be used to reboil a fraction of the lean absorbent in order to decrease the overall energy consumption of the carbon dioxide recovery process.
However, in large facilities, even a minor decrease in the energy consumption per unit of carbon dioxide yield is of great economical benefit. Therefore, there is an ongoing need for designing processes and parameters resulting in the recovery of carbon dioxide at lower energy expenditure. In addition to energy expenditure there is an equally important economic aspect in reducing other resources consumed per unit carbon dioxide yield, such as the amount of absorbent and/or water required in the process.
Hence, it is an object of the present invention to further reduce the overall energy consumption of acid gas recovery processes as well as reducing the consumption of additional resources such as water, and it is a further object to provide a process with flexibility in the energy consumed for the stripping process.